Keeping in step with the recent marked developments in information equipment, liquid-crystal color displays have found wide-spread utility in information-display-related equipment as information display units, for example, as display screens in personal computers, mobile information equipment, television sets, projectors, monitors, car navigation systems, cellular phones, electronic calculators and electronic dictionaries, as displays such as information display boards, guidance display boards, function display boards and sign boards, and also as monitor screens in digital cameras and video cameras. As a corollary to this, color filters to be incorporated in liquid color displays are now required to meet still higher qualities in image characteristics such as fineness or definition, color density, light transmittance, and contrast.
The color tones of three primary-color pixels, specifically red (which may hereinafter be abbreviated as “R”) pixels, green (which may hereinafter be abbreviated as “G”) pixels and blue (which may hereinafter be abbreviated as “B”) pixels on a color filter for use in a liquid color display are required to conform with the corresponding maximum emission wavelengths (for example, R: 610 nm, G: 545 nm, B: 435 nm) in the energy distribution of a three-wavelength fluorescent lamp used as an illustrative backlight. The G pixels are required to have a transmission wavelength in conformity with the maximum transmission wavelength (=545 nm) of the G color of the three-wavelength fluorescent lamp, to have a high transmittance for the maximum transmission wavelength, and to block R and B light emissions.
Green pigments commonly employed as colorants in synthetic resins, printing inks and the like include C.I. Pigment Green (hereinafter abbreviated as “PG”) 7 (copper polychlorophthalocyanine pigment) and PG36 (copper polybromopolychlorophthalocyanine pigment). As a green pigment for the formation of G pixels on color filters, mainly used is PG36 which shows a yellowish green color. PG36 is, however, obtained by bromination and chlorination of unsubstituted copper phthalocyanine. Accordingly, the numbers of substituted bromine atoms and chlorine atoms vary from one production batch to another as will be mentioned subsequently herein. G36 is, therefore, accompanied by problems in that its color tone does not remain constant in a yellowish green color suited for color filters and green pigments insufficient in a tincture of yellow are formed abundantly.
To effectively exhibit the optical characteristics of G pixels, a yellow pigment is added in a proportion of from 20 to 100 wt. % based on PG36 to block the transmission of light in a short wavelength range through PG36. The color tone (yellowish green color) of PG36, however, tends to vary from one production lot to another as described above. A yellow pigment is, therefore, added as much as 30 to 120 wt. % based on PG36 to correct the maximum transmission wavelength of the resulting G pixels to 545 nm. As a result, the G pixels are accompanied by problems in that their transmittance is sacrificed, and due to the addition of the yellow pigment in such a large proportion, neither the hue nor the chromaticy is produced as expected in some parts of a chromaticy diagram.